The PCM transmission system has widely been studied because of its many advantages on the picture transmission, and its typical examples are the DPCM coding system and the orthogonal transform coding method, both of which utilize the statistic property of a picture.
The DPCM method makes use of statistic and visual properties of a level variation but is considered to have a difficulty in that an abrupt level variation and a gentle slope of a picture, which are extremely different in nature from each other, are processed by a certain signal operation.
Meanwhile, the orthogonal transform coding system starts from an idea of determining an orthogonal transform in such a manner as to minimize a square mean error by a linear operation based on a correlation matrix of a picture and is the most effective with respect to a continuous Gaussian process or the like. However, a picture has a great feature in a contour portion having abrupt level variations. In this point, the orthogonal transform presents a problem.
Further, when the picture is subjected to Hadamard transform, not only Gaussian noise but also irregular movements of an oblique straight contour portion, and defocusing of a contour out of synchronization with an orthogonal function may appear in some cases, making the picture unnatural.